The semiconductor industry uses perfluorocompounds such as CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.8, C.sub.4 F.sub.10, CHF.sub.3, SF.sub.6, NF.sub.3, and the like, in semiconductor manufacturing processes involving gases, particularly in various etching steps of the semiconductor manufacturing processes as well as in the chamber cleaning step of the manufacturing process. Such perfluorocompound gases are used either pure or diluted, for example with air or nitrogen or other inert gas or in admixture with other perfluorocompound gases or other carrier gases (for example inert gases). All of the perfluorocompound gases do not necessarily react with other species during the manufacturing processes. Further, when reactors are cleaned or evacuated to carry out another step of the manufacturing process, the effluent gases or gas mixtures are preferably not vented, even if they are largely diluted with air or any other gas such as inert gas. Most of the perfluorocompounds (also called "PFCs") have lifetimes measured in thousands of years in the atmosphere and are also strong infrared absorbers. In the "Global Warming Symposium" held on Jun. 7-8, 1994, in Dallas, Tex., U.S.A., carbon tetrafluoride (CF.sub.4), hexafluoroethane (C.sub.2 F.sub.6), nitrogen trifluoride (NF.sub.3), and sulfur hexafluoride (SF.sub.6) were identified as "greenhouse gases" of concern to the semiconductor industry.
In a presentation made at the above symposium by Michael T. Mocella entitled "Perfluorocompound Emission Reduction From Semiconductor Processing Tools: An Overview Of Options And Strategies", various possible strategies to control emission of these gases in the atmosphere were presented.
Apart from process replacement by non PFCs, several methods are already known or under development:
chemical-thermal decomposition with various activated metals wherein the spent bed material must be disposed. This method is presently considered commercially promising but unproven technology. PA1 combustion-based decomposition process (or chemical-thermal process) using a flame to supply both the thermal energy, and the reactants for the decomposition. Here, there are some safety issues associated with the hydrogen or natural gas fuels used and all the PFCs will produce hydrofluoric acid (HF) as a combustion product (if the temperature is high enough), whose emissions must also be abated. It was suggested that decomposition temperatures may also be generated using a resistance heater. PA1 plasma-based decomposition process which involves the use of a plasma such as coupled radio frequency systems to partially decompose C.sub.2 F.sub.6, with over 90% decomposition of C.sub.2 F.sub.6. However, such systems are not yet commercially proven. Oxygen is usually needed to drive the decomposition to non PFC products, and the problematic generation of HF would be present. PA1 recovery process wherein the PFCs are recovered instead of being destroyed in order to be recycled. This kind of process is of a great interest because it is considered as an environmentally responsible approach. Different schemes were suggested as possible "based on combinations of adsorption or low temperature trapping of PFCs". There are, however, several challenges such as dealing with the large amount of nitrogen associated with the pump operation, the close boiling points of CF.sub.4 and NF.sub.3, the mixing, of various process streams and/or possible reactions with adsorbents. While recycle was suggested, there are obvious questions about recycling such mixtures. PA1 a) providing a gas mixture comprising at least one perfluorocompound gas and at least one carrier gas, the gas mixture being at a predetermined pressure; PA1 b) providing at least one membrane having a feed side and a permeate side, the membrane exhibiting preferential permeation of at least one carrier gas and being relatively non-permeable to at least one perfluorocompound gaseous species and for which selectivity SEL is greater than 1.0. PA1 wherein, SEL is D.sub.c ! S.sub.c !/D.sub.p ! S.sub.p ! PA1 c) contacting the feed side of the at least one membrane with the gas mixture; PA1 d) withdrawing from the feed side of the membrane as a first non-permeate stream at a pressure which is substantially equal to the predetermined pressure, a concentrated gas mixture comprising essentially at least one perfluorocompound gas, and PA1 e) withdrawing from the permeate side of at least one membrane as a permeate stream a depleted gas mixture consisting essentially of the at least one carrier gas. PA1 a) providing a glassy polymer membrane having a feed side and a permeate side; PA1 b) providing a gas mixture at a first pressure comprising at least one perfluorocompound gaseous species, at least one harmful species for the membrane, and at least one carrier gas; PA1 c) treating said gas mixture with a dry scrubber and/or wet scrubber, or contacting in scrubber means to substantially remove species harmful to said membrane and reduce the concentration of said harmful species to an acceptable level for said membrane resulting in a scrubbed gas mixture at a second pressure; PA1 d) contacting the feed side of said membrane with said scrubbed gas mixture at substantially said second pressure or at a higher pressure; PA1 e) withdrawing a concentrated gas mixture comprising a higher concentration of the at least one perfluorocompound gas, relative to the scrubbed gas mixture, from the feed side of the membrane as a non-permeate stream at a pressure which is substantially equal to said second pressure, and PA1 f) withdrawing a depleted gas or gas mixture from the permeate side of said membrane as a permeate stream which is enriched in a carrier gas and depleted in the at least one perfluorocompound. PA1 a) at least one reactor chamber adapted to receive perfluorocompound gases, carrier gases, and the like, the reactor chamber having a reactor effluent gas conduit attached thereto; PA1 b) at least one size selective, preferably glassy polymer, membrane separation unit having a feed side and a permeate side, the membrane being preferentially permeable to at least one carrier gas and being substantially non-permeable to at least one perfluorocompound gas, the membrane unit connected to the reactor chamber via the reactor effluent conduit, the membrane unit further having a permeate vent conduit and a non-permeate conduit, the latter adapted to direct at least a portion of a perfluorocompound containing non-permeate stream from the membrane unit to the reactor chamber. Preferred systems in accordance with the invention further provide pretreatment and/or post-treatment means, such as dry or wet, (or both) scrubbers, thermal decomposers, catalytic decomposers, plasma gas decomposers or filters, prior to the reactor effluent stream entering the membrane unit. In another embodiment, a plurality of membrane units may be arranged in series, either with or without provision of sweep gas on the permeate side of one or all membranes.
Another combustion system for destroying high nitrogen content effluent gas streams comprising PFCs is disclosed in an article entitled "Vector Technology's Phoenix Combustor" by Larry Anderson also presented at the same symposium Jun. 7-8, 1994. This abatement system also uses a gas flame (natural gas or hydrogen with air), which leads then to the same problem of HF generation together with the need for further destruction (plus the Generation of NO.sub.x, CO.sub.2 inherent to any combustion process).
In the article presented at the same symposium Jun. 7-8, 1994 by AT&T Microelectronics and Novapure Corporation entitled "PFC Concentration and Recycle", the authors acknowledge the advantages of the recovery processes which avoid production of carbon dioxide, NO.sub.x, and HF (compared to combustion processes). The process disclosed the use of a dual bed adsorber (activated carbon), wherein one of the beds is in the adsorption mode, while the second bed is regenerated: the PFCs are adsorbed on the carbon sieves while the "carrier" gases, such as nitrogen, hydrogen are not adsorbed and are vented to the exhaust system. When the system is switched on the second adsorber, the first adsorber is evacuated using a vacuum pump, and the effluent is recompressed and the PFC gas mixture is recovered. One of the issues not yet resolved with such a system is that CF.sub.4, which is non polar, is not readily adsorbed by the carbon sieve, but is then rejected with the vent gases. Also, any adsorption system is very sensitive to moisture and any trace of water has to be removed from the feed flow.
It is known from U.S. Pat. No. 5,281,255 incorporated herein by reference, to use membranes made of rubbery polymers such as poly dimethyl siloxane or certain particular polymers such as a substituted polyacethylene to recover condensable organic components having a boiling point higher than -50.degree. C., essentially hydrocarbons (CH.sub.4, C.sub.2 H.sub.6, and the like), said hydrocarbons having the property of permeating through said membranes much faster than air, and then recovering on the permeate side of the membrane said hydrocarbons. The permeate (hydrocarbons) is then recovered at either substantially atmospheric pressure or lower pressure while the non-permeate (e.g. N.sub.2) remains at the feed pressure and is vented. However, by this approach, all of the pressure energy of the feed stream is lost.
Also, it is disclosed in WO 90/15662, published Dec. 27, 1990, a selectively permeable membrane formed from an amorphous polymer of perfluoro 2-2 dimethyl 1-3-dioxole which is useful in the separation of hydrocarbons or chlorofluorocarbons from, for example, air. Such a particular membrane apparently permeates oxygen and nitrogen faster than hydrocarbons and chlorofluorocarbons which can be recovered unexpectedly on the non-permeate side of the membrane, contrary to all of the membranes, including those disclosed in U.S. Pat. Nos. 4,553,983 and 5,281,255. In this PCT application, there is also disclosed a mixture of the amorphous polymer of perfluoro 2-2 dimethyl 1-3 dioxole and polytetrafluoroethylene. All these perfluoro polymers are known to be resistant to most of the harmful chlorofluorocarbons and hydrocarbons which might suggest their commercial suitability for such separation. However, such polymer is not well suited for PFC recovery, having only limited utility at low concentration.
There exists a need for an environmentally sound process for concentration and/or recovery of PFCs from a gaseous stream, which can be used with a feed flow comprising or saturated with, moisture, which can safely handle recovery and/or concentration of PFC's even with important or extreme variations of flows and/or concentration and which does not produce hydrofluoric acid (HF) as a by product.